Medical Pharmacology Chapter 12: Anxiolytics and Sedative-Hypnotics
Antacids are exceedingly effective in increasing gastric fluid pH to > 2.5 when administered 15-30 minutes prior to anesthesia induction
An important factor in the affects the antacid efficacy is simply the extent of patient movement-with increased movement promoting more complete antacid mixing with the gastric fluid
Concerns associated with inhalation of gastric fluid containing antacids:
If aspiration does occur and gastric fluid containing antacid particulates is inhaled a possible significant inflammatory reaction may ensue which can cause significant pulmonary dysfunction.
This possibility is an argument in favor of non-particulate antacids such as sodium citrate (0.3 M) which also is effective in raising gastric fluid pH to > 2.5 without producing significant pulmonary complication should inhalation of fluids occur.
Pulmonary complications can include pulmonary edema and arterial hypoxemia.
Special advantages of antacids compared to H2 receptor blockers:
Administration of an antacid immediately increases gastric pH, without the lag time associated with histamine receptor blockers.
The antacids, however, may increase gastric fluid volume, although this effect should not be interpreted as to discourage antacid use and is more likely to occur after repeated doses (such as during labor) and especially if opioids have been given which themselves delay the gastric emptying
As noted earlier for the receptor blockers, antacids need not be routinely used, but rather used for those selected patients who appear to have a higher risk for pulmonary aspiration.
3Rationale: A large percentage of patients will exhibit gastric fluid volumes > 0.4 ml/kg with a pH < 2.5 (about 3/4 of pediatric patients and about half of adult out-patients qualify)
However the likelihood of aspiration turns out to be very low.
In one study of about 40,000 anesthetic procedures in children, aspiration was noted only four times (2 occasions intraoperatively and two occasions postoperatively). [Tiret, L., Nivoche, Y, Hatton, F, et al: Complications related to anesthesia in infants and children: a prospective survey of 40,240 anaesthetics. Br J. Anaesth 1988: 61: 263]
In a separate study involving retrospective assessment of 185,000 procedures, 83 aspiration occurrences were noted resulting in an incidence rate of about 1 in 2000. [Olsson, GL, Hallen, B. Pharmacological evacuationof the stomach with metoclopramide. Acta Anesth Scand, 1982; 26, 417]
For most of these cases (68 out of 83), other factors which would be expected to delay gastric emptying were noted. Therefore, an approach that involves routine premedication to avoid aspiration pneumonitis is difficult to support; however identification of risk factors IS important.
3Risk factors for aspiration pneumonitis: causative factors for gastric emptying delay:
Elevated intracranial pressure
Gastritis or ulcer history
Emergency abdominal surgery
Elective upper abdominal surgery
3Patients who should receive aspiration prophylaxis: (Table 34-6)
Anticipated challenging airway intubation
Emergency surgical patients
Patients exhibiting reduced level of consciousness which may be caused by head trauma or drug overdosage
Elevated intracranial pressure due to mass effects or edema
Impaired laryngeal reflexes which could be caused by Shy-Drager syndrome (multiple system atrophy), amyotrophic lateral sclerosis (Lou Gehrig's disease), vocal cord paralysis, stroke, bulbar palsy .
Bulbar palsy: bulbar refers to the lower brain stem, i.e. cranial nerves 7-12; palsy refers to weakness.
Therefore bulbar palsy refers to weakness of muscles controlled by cranial nerves 7-12.
Manifestations could include difficulty in speaking, swallowing, coughing as well as difficulty with facial expressions.
Bulbar palsy, therefore, may be manifestation of a number of diseases including Lou Gehrig's disease, stroke, or inflammatory disease.
Ulcer disease including previous surgeries such as partial gastrectomy or vagotomy the latter of which would lead to gastroparesis due to the absence of cholinergic tone following nerve section.
Hiatal hernia and reflux
hiatal hernia definition: A hiatal hernia could be defined as a condition in which a portion of the stomach pushes through the diaphragm into the chest cavity. This condition is relatively common affecting about 15% of the population (U.S.)
Reflux and hiatal hernias: The size of the hiatal hernia predicts the likelihood of symptoms.
For large hernias the symptoms are almost always associated with gastro-esophageal reflux disease or GERD
GERD occurs because the hernia itself interferes with the lower esophageal sphincter which usually prevents gastric acid from refluxing into the esophagus. GERD can occur in the absence of a hernia.
Gastric acid typically does not reflux into the esophagus for a couple of reasons
(1) because the diaphragm muscle wraps around the region of the lower esophageal sphincter.
Therefore both the diaphragmatic muscle and the lower esophageal muscle contribute to blocking regurgitation.
In the presence of a hiatal hernia, says the lower esophageal sphincter is no longer in the region the diaphragm, the diaphragmatic pressure component is lost, making regurgitation more likely.
(2) The second reason is that normally the esophagus enters the stomach at an angle, a fairly sharp angle, with a thin piece of tissue at this location forming a "valve".
When the stomach protrudes through the diaphragm in hiatal hernia, the sharpness of the angle between the esophagus and stomach is significantly reduced with the concurrent reduction in the ability of the valve to prevent regurgitation.
Upper abdominal surgery
3Gastric paresis (paralysis) caused by other sources including dialysis or diabetes.
3Choice of agents for prophylaxis of expected aspiration:
For trauma patients, sodium citrate (30 ml-raises the pH of gastric fluid already present); ranitidine (Zantac) by IV administration at a dosage of 50 mg; metoclopramide (Reglan), 20 mg by IV administration to facilitate gastric emptying.
For preparation of elective surgery patients who have a difficult airway:
Ranitidine (Zantac) orally administered at an appropriate dosage at 7 p.m. and in the morning of surgery; metoclopramide (Reglan) at an appropriate dosage of administered orally in the morning of surgery; glycopyrrolate (Robinul) at an appropriate dose for IV administration to reduce secretions in support of fiberoptic bronchoscopy.
1Preoperative Medication in Basis of Anesthesia, 4th Edition, Stoelting, R.K. and Miller, R., p 119- 130, 2000)
Hobbs, W.R, Rall, T.W., and Verdoorn, T.A., Hypnotics and Sedatives; Ethanol In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc., 1996, pp. 364-367.
3Sno E. White The Preoperative Visit and Premedication in Clinical Anesthesia Practice pp. 576-583 (Robert Kirby and Nikolaus Gravenstein, eds) W.B. Saunders Co., Philadelphia, 1994
4John R. Moyers and Carla M. Vincent Preoperative Medication in Clinical Anethesia, 4th edition (Paul G. Barash, Bruce. F. Cullen, Robert K. Stoelting, eds) Lippincott Williams and Wilkins, Philadelphia, PA, pp 551-565, 2001
5Michael Ross and Susan Dufel "Torticollis" emedicine, http://www.emedicine.com/emerg/topic597.htm